We use our heated propagator for starting off things like chillies which require high germination temps and can also take a while to start... and tomatoes - mainly early part of the year, when things (soil temp) is too low.

Looking at that link you posted, perhaps you might want to consider a new propagator with integral temp sensor thermostat (given the price for that Habistat).

As for worrying about sunlight - if you are in the UK, then not too much to worry about at the moment - I just read that they are now talking about the wintry weather lasting until late April.

As for tips - beware starting things off in your propagator too early if you don't have anywhere for them to be moved to. Also, once seeds have germinated, they do need light... and I don't know where you are, but there is not much beyond greyness where we are! Also watch out for hardening off once you take seedlings out of their cozy propagator.

I considered a propagator with a built-in thermostat, but I
can’t find a reasonably sized one which would comfortably fit on my window sill
for under £100. Hence why I would like to modify my existing one if possible.

I have a digital thermometer standing on the heated surface
of my propagator which I try to keep reading 22 degrees – I’m growing Tomatoes
and chillies – and somewhere I read that 22 was the optimum temperature for
tomatoes. When the sun makes it’s incredibly rare appearances, or the central
heating in the house kicks in, the thermometer reading rockets to 27 or more.
And I switch the propagator off. Then the sun vanishes- again – or the house cools, and the
temperature plummets to 15 or less. So I switch it on again. I’m starting to
feel like a living thermostat myself!

Last year I managed to kill my tomato seedlings by going
out, leaving the propagator switched on, and the sun coming out.

I could probably buy the components quite cheaply and
build a thermostatic switch linked in to a temperature sensor, but I’m not
confident of what to do, and I don’t fancy burning the house down. (I know some
seeds require fire to germinate, but I’m fairly sure that tomatoes and chillies
aren’t amongst them ;-).Which is why I
was considering the Habistat.

You only need to use the propagator until the seedlings emerge, then as soon as they emerge, take them out of the propagator and they will be perfectly fine on the windowsill itself. Tomato and chilli seedlings will happily sit on a windowsill or other light place in 10°C so no need to be forking out for something that is unneccessary.

Sue is right about not leaving things in the propagator; we normally wait until things have at least their first proper leaves (i,e, not the seed leaves) and there is often a queue for the next things to be germinated.

We have found that some chilli seeds will happily germinate around lower temps, but we had some that just sat there until we cranked the thermostat upto 25+.

Well, I did it. I bought a habistat thermostat and have been using it with my propagator for the last couple of days. It works well. However, thrifty? Maybe not entirely. If you are buying a new heated propagator, I would recomend you go for one with a thermostat in the first place - don't do what I did and buy the cheapest one available.

But... if you are upgrading an existing one which just happens to fit the space you have available nicely, it works well, and is cheaper than a new propagator. And it certainly beats actively monitoring the temperature, and fretting when you go out - honestly, you'd think I had a puppy, not a bunch of seedlings!